In recent years, the so-called press-hardening technology has gained ever-increasing importance in car body construction.
Initial developments of this press hardening process from the 1970s involved the heating of flat sheet metal blanks and the shaping and simultaneous cooling of heated sheet metal blanks in a single, cooled tool. In this connection, the sheet metal blank is heated to a temperature above the AC3 point and is thus partially or fully transformed into austenite. The quench hardening of the austenitic structure causes a martensitic hardening of the sheet metal component.
This press hardening method only became economically significant much later when it became necessary to produce vehicle bodies and in particular passenger compartments that were much more stable and rigid. The high levels of hardness that can be achieved with the press hardening method are advantageous in this regard.
In the course of further development, however, it has turned out that components that are consistently very hard, e.g. longitudinal beams, B pillars, cross members, etc. that demonstrate hardly any deforming behavior, are not ideal. Instead, it has since become necessary for particular regions of the component to be very hard while other regions are more ductile in order to permit a certain amount of deformation so as to prevent, for example, the component from fracturing.
It was also necessary not only to be able to manufacture such components without a coating, but also for them to be coated in a manner adapted in accordance with a corrosion protection coating of the entire body. In particular, it has become necessary to provide high-strength galvanized components. Basically, the press hardening method is divided into the so-called direct and indirect methods.
In the direct press hardening method, a flat blank is correspondingly heated to a temperature above the Ac3 temperature of the respective steel compound, is kept there for a desired span of time, and is then shaped by means of a single shaping stroke in a tool and, because the tool is simultaneously cooled, is cooled and hardened with a cooling speed that is greater than the critical hardening speed.
In the indirect method, the blank has already been shaped into the finished component, then the finished component is heated to a temperature above the Ac3 temperature of the respective steel compound, possibly kept at this temperature for a predetermined time, and then transferred to a corresponding forming tool, which likewise has the contour of the finished component, and once there, is cooled and hardened by this tool.
The advantage of the direct method is the relatively high cycle rates, but the single shaping stroke and the material behavior in the hot state make it possible to achieve only relatively simple component geometries.
The advantage of the indirect method is that it is possible to produce very complex components since the component itself can be shaped with any number of shaping strokes in the contour shaping appropriate to the manufacture of a normal body component. The disadvantage is a slightly lower cycle rate. But the indirect method has the advantage that a shaping step no longer occurs in the heated state, which is advantageous particularly with the use of metallic coatings because the metallic coatings are frequently in a partially liquid form at the high temperatures for the austenitization. In connection with the existing austenite, these liquid metallic coatings can result in a crack formation due to so-called “liquid metal embrittlement.”
EP 1 651 789 B1, which belongs to the applicant, has disclosed a method for producing hardened components from sheet steel in which the shaped parts are cold-formed out of a sheet steel that is provided with a cathodic corrosion protection and then a heat treatment is carried out in order to achieve austenitization; a final trimming of the shaped part, required punching operations, and the production of a hole pattern are carried out before, during, or after the cold-forming of the shaped part; the cold forming, the final trimming, the punching, and the production of a hole pattern in the component are carried out in such a way that the shaped part is 0.5% to 2% smaller than the final hardened component so that trimming is no longer required in the hard state.
DE 10 2004 038 626 B3 has disclosed a method for producing hardened components from sheet steel in which the shaped parts are formed out of a sheet steel and a required final trimming of the shaped part and possibly required punching procedures for producing the hole pattern are carried out before, during, or after the shaping of the shaped part; finally, at least some regions of the shaped part are heated to a temperature that permits the steel material to austenitize and the component is then transferred to a form hardening tool and in the form hardening tool, a form hardening is carried out in which the component is cooled and thus hardened by the fact that the form hardening tool contacts and presses against the component, at least in some regions; the component is supported by the form hardening tool in the region of the positive radii and in at least some regions and in the region of the trimming edges, is held in a clamping, distortion-free fashion; in the regions in which component is not clamped, the component is spaced apart from at least one of the forming tool halves, leaving a gap between them.
DE 10 2005 057 742 B3 has disclosed a method for heating steel components in which the steel components to be heated are conveyed through a furnace and in the furnace, are heated to a predetermined temperature; a transport apparatus for transporting the components through the furnace is provided; a first transport device takes up the components in a precisely positioned fashion transports them through the furnace to heat them and after the heating, a second transport device takes the parts from the first transport device at a predetermined transfer point or transfer region and then conveys them out of the furnace at an increased speed and in a precisely positioned fashion, delivers them to another transfer point for further processing; the cited patent has also disclosed a device for heating steel components.
DE 10 2008 063 985 A1 has disclosed a method for producing a hardened sheet metal component from a sheet steel in which a sheet steel blank or a preformed or completely formed sheet steel component is heated to a temperature required for hardening and is then inserted into a tool in which the blank or the sheet steel component is hardened. In order to produce areas with less hardening or without hardening, the tool has recesses that are flushed with gas in this region; this gas flushing is carried out so that in these regions, gas cushions are produced whose presence reduces or prevents a cooling at a speed greater than the critical hardening speed; the cited patent has also disclosed a device for carrying out the method.
WO 2006/038868 A1 has disclosed a press hardening method in which a blank is formed and cooled in a cooled tool and in which the tool is used as a fixing device during the hardening. To this end, the tool has alternating contact surfaces and recesses that press against the shaped product in a particular region; the contact regions make up less than 20% of the total surface area. As a result, this region should be a soft zone of the final product and should nevertheless have a good dimensional accuracy.
DE 10 2007 057 855 B3 has disclosed a method in which a blank produced from a coated, high-strength boron steel is homogeneously heated to a temperature of approximately 803° C. to 950° C. in a furnace having several temperature zones and is kept at this temperature level for a certain amount of time. Then a first-type region of the blank is cooled to a temperature of approximately 550° C. to 700° C. in a second zone of the furnace and is kept at this reduced temperature level for a certain amount of time. At the same time, a second-type region of the blank is kept at a temperature level of approximately 830° C. to 950° C. in a third zone of the furnace for a certain amount of time. After this heat treatment, the blank is shaped into a shaped component in a hot-forming process. In this case, the component should be embodied with an aluminum/silicon coating; in the way described above, the first-type regions and second-type regions of the shaped part should have different ductility properties.
DE 10 2006 006 910 B3 has disclosed a body frame structure or running gear structure that is composed of steel structural components in which at least the load-bearing steel structural components should have zinc plate coatings that function as a corrosion protection coating.
DE 10 2004 007 071 A1 has disclosed a method for producing a component by shaping a coated blank that should be composed of a tempering steel; before the shaping, the blank is austenitized through a first heat treatment and should undergo a growth in layer thickness. The process should be optimized in that after a rapid cooling, the heat-treated blanks are temporarily stored; just before being shaped into the component, the blank undergoes a brief additional heating to the austenitization temperature and after the structural change has occurred, the shaping and hardening of the blanks should take place. The heating should preferably take place by means of induction.
DE 10 2005 014 298 A1 has disclosed an armoring for a vehicle; the armoring is produced by means of hot forming and press hardening; the intent of this is to enable the production of complex armors with a matching contour to be produced with a small number of welding seams.
DE 10 2009 052 210 A1 has disclosed a method for producing components out of sheet steel with regions of different ductility; either a sheet metal blank made of a hardenable steel alloy is used to produce a component by means of deep drawing and the deep-drawn component is then at least partially austenitized through a heat treatment and then quench-hardened in a tool or the blank is at least partially austenitized through a heat treatment and formed in a hot state and in the course of this or subsequently, is quench hardened; the sheet metal blank has a zinc-based cathodic corrosion protection coating; in regions of a desired higher ductility of the component, at least one other sheet is placed onto the blank so that during the heat treatment, the blank is heated to a lesser degree there than in the remaining region.
DE 10 2006 018 406 A1 has disclosed a method for heating work pieces, in particular components provided for press hardening; the work piece is supplied with heat for a period of time in order to heat it to a predetermined temperature, then during the heating, heat is conveyed away from a selected section of the work piece so that the temperature reached in the selected section during the heating period lies below the predetermined temperature. For example, the predetermined temperature is the temperature required for an austenite structure to form during the press hardening. In this case, the work piece is placed in a continuous furnace for heating and rests with selected sections against a respective body. The bodies are components of a tool mount—not otherwise shown—that can be moved into and out of the continuous furnace. The work piece can also be a preformed sheet metal component. The heat-absorption capacity of the bodies resting against the sections of the work piece is dimensioned so that up to the end of the heating time, the temperature of these bodies only reaches a value below the above-mentioned temperature threshold so that during the heating of the work piece, heat partially flows into the bodies. Before the mount is reused, the bodies cool down to a predetermined starting temperature or are cooled by means of a coolant.
DE 200 14 361 U1 has disclosed a B pillar for a body component, which is composed of a longitudinal profile made of steel; the longitudinal profile has a first longitudinal section with a predominantly martensitic material structure and a second longitudinal section with a higher ductility and a predominantly ferritic material structure; the different structures are produced so that during the heating of the component or blank, a protective or insulating body covers the region that should not be heated as intensely.
DE 10 2009 015 013 A1 has disclosed a method for producing partially hardened steel components in which a blank composed of a hardenable sheet steel is subjected to a temperature increase sufficient for a quench hardening and after reaching a desired temperature and possibly after a desired sojourn time, the blank is transferred into a forming tool in which the blank is shaped into a component and is simultaneously quenched or else the blank is cold formed and the component produced by the cold forming is then subjected to a temperature increase; the temperature increase is carried out so that a temperature of the component is reached which is required for a quench hardening and the component is then transferred to a tool in which the heated component is cooled and thus quench hardened; during the heating of the blank and component to increase their temperatures to a temperature that is required for the hardening, one or more absorption masses rest against regions that are intended to have a lower hardness and/or high ductility; with regard to its size and thickness, its thermal conductivity, and its thermal capacity, each absorption mass is dimensioned so that the thermal energy acting on the component in the region that remains ductile flows through the component into the absorption mass.
DE 10 2008 062 270 A1 has disclosed a device and corresponding method for partially hardening a metallic work piece; a conveying device transports the work piece in a conveying direction in a continuous furnace and is partially heated by means of a heating device; the heating device produces at least one heating zone that is moved in the conveying direction along with the work piece. In this way, the heating zone provided by the heating device can travel along with the work piece being continuously moved in the conveying direction so that only the section situated in the heating zone, but not the sections of the work piece situated outside the heating zone can be heated to a predetermined temperature, for example to the so-called austenitization temperature of steel.
DE 10 2008 030 279 A1 has disclosed a hot-forming line, which is intended to enable production of a partially hardened steel component through the processing that is carried out in several successive stations. During the production of the partially hardened component, it is, among other things, homogeneously heated to a temperature <AC3 in a heating station in order to then be conveyed under an infrared lamp station and in the latter, is heated to a temperature above AC3 in only some regions. In this way, the steel component is only partially hardened in the subsequent cooling process.
The object of the invention is to create a method for producing a partially hardened steel component with which it is possible to heat and produce such components quickly, inexpensively, and with high precision.
Another object of the invention is to create a device for carrying out the method, which has a simplified design, permits a high throughput capacity, enables a precise partial heating, and is also energy efficient.